Field of the Invention
The invention relates to methods and devices for the highly sensitive acquisition of high resolution ion mobility spectra.
Description of the Related Art
In document U.S. Pat. No. 7,838,826 B1 (M. A. Park, 2008), a small ion mobility spectrometer is presented. The length of the essential ion mobility scanning part, the ion mobility scanning tunnel, amounts to about five centimeters only; with additional entrance and exit funnels the device is less than ten centimeters long. The cylindrical ion mobility scanning tunnel comprises a quadrupolar RF field keeping the ions near the axis; separation of ions according to their mobilities is based upon a gas flow in the tunnel, driving the ions in an accumulation phase from an ion source against a ramp of an electric DC counter-field barrier. After shutting down the delivery of further ions, a scan phase starts, in which the field barrier is steadily decreased. Ions are driven in this scan phase by the gas flow over the decreasing top of the field barrier, thereby releasing successively ions from low mobilities to higher and higher mobilities from being trapped by the barrier. The ions can be detected in an ion detector, resulting in a mobility spectrum. Particularly, the ions may be measured by a mass spectrometer, e.g. a time-of-flight mass spectrometer, resulting in a two-dimensional mass-mobility map. The advantage of this tiny mobility spectrometer over all other mobility spectrometers is the possibility to choose a wanted mobility resolution by choosing the right scan speed. With extremely low scan speeds, the device by M. A. Park has already achieved ion mobility resolutions up to Rmob=400, a very high ion mobility resolution never achieved by any other mobility spectrometer. The device became widely known as “TIMS”, “trapped ion mobility spectrometer”. The principle of the device and its basic operation is outlined in FIG. 1.
In FIG. 2, the ion mobility resolution of this instrument is plotted as function of the time needed for a scan over the usual range of ion mobilities. With a scan time of only 20 milliseconds, a mobility resolution of about Rmob=60 is achieved, with 300 milliseconds scan time, the resolution amounts to Rmob=120.
The document U.S. Pat. No. 8,766,176 B2 (“Acquisition Modes for Ion Mobility Spectrometers using Trapped Ions”, D. A. Kaplan et al.; 2011) presents a variety of scan modes for TIMS, e.g. a scan mode to achieve a linear mobility scale. A special scan mode concerns a “temporal zoom mode”, wherein the scan runs with high speed through a first range of ions with low mobilities, more or less to get rid of the ions, traverses a second range of mobilities with low speed to measure the mobilities of the ions in this range with high ion mobility resolution, and passes through the end of the mobility spectrum with high speed again. This procedure saves time, if only a small range of mobilities is to be measured with highest mobility resolution. Such a temporal zoom mode is not achievable with drift tubes or any other type of mobility spectrometers.
U.S. patent application Ser. No. 14/614,463 (“High Ion Utilization Ion Mobility Separator for Mass Spectrometers”, M. A. Park and O. Räther) illustrates how the influence of space charge in selected mobility ranges can be diminished by flattening the ramp of the electric barrier. If the ramp is made flatter in a certain range of the mobility spectrum, the ion clouds of different mobilities within this range are spatially uncompressed, reducing the influence of space charge causing ion losses due to Coulomb forces. The effect of non-linear electric field ramps on the density of accumulated ions is illustrated in FIG. 3 showing a decompression of high mass ions (low mobility). High mass ions are highly endangered to be piled out of the tunnel by space charge forces because they are only weakly bound inside the tunnel by the quadrupolar RF field. Experiments proved that losses of heavy ions of interest are greatly diminished.
U.S. patent application Ser. No. 14/614,456 (“Trapping Ion Mobility Spectrometer with Parallel Accumulation”, M. A. Park and M. Schubert) presents an ion mobility spectrometer of the type described in the various documents cited above, additionally equipped with an extra upfront accumulation unit, the accumulation unit having the same form as the scanning unit. The accumulation unit operates in parallel with the scan by the separator tunnel. That is, while the separator is being used to analyze the mobilities of a first group of ions, the accumulation unit is simultaneously collecting a second group of ions from an ion source. This second group of ions is then rapidly transferred—in about a millisecond—to the ion mobility scanning tunnel once the analysis of the first group is complete. This allows the ion accumulation unit to collect ions nearly continuously while the separator tunnel analyzes ions nearly continuously in a repetitive way. The utility rate of ions generated in the ion source and transferred into the vacuum system is nearly 100 percent. An example of the device with an ion accumulation tunnel and an ion mobility scanning tunnel and its operation is shown in FIG. 4. In both the ion accumulation tunnel and the ion mobility scanning tunnel, the ions are gathered on an electric field ramps, separating the ions according to their mobility and diminishing the deteriorating effect of space charge on the ion collection causing ion losses.
There is still a need for devices and methods for the acquisition of mass-mobility maps with highest utility rates of the ions generated in an ion source of a mass spectrometer, with range of ion mobilities, mobility separation time and mobility resolution adaptable to the requirements of a given analytical task.